טבילה בזמנה מצוה The Mitsvah (?) Of Timely Immersion

...In that case, the Rabbis hold that performing immersion at its designated time is not a mitzva, and we seek a reed to wrap around God’s name even if it means postponing immersion to the next day, and Rabbi Yosei holds that immersion at its designated time is a mitzva, and therefore we do not seek a reed, since immersion cannot be postponed. The Gemara asks: And does Rabbi Yosei hold that immersion at its designated time is a mitzva? Wasn’t it taught in a baraita: With regard to a zav and a zava, a male and female leper, one who has relations with a menstruating woman, and a person impure with impurity imparted by a corpse, their immersion is during the day. They immerse at the designated time even on Yom Kippur, when bathing is prohibited. A menstruating woman and a woman after childbirth immerse at night. A man who has had a seminal emission immerses at any point during the entire day after the emission. Rabbi Yosei says: From the time that he recited the afternoon prayer and on he does not immerse. Since he already recited the afternoon prayer, he waits until after Yom Kippur to immerse, and then recites the evening prayer in a state of purity. Apparently, Rabbi Yosei holds that immersion at the designated time is not a mitzva. The Gemara rejects this: In that baraita the reference is not to the Rabbi Yosei most commonly cited in tannaitic literature without a patronymic, Rabbi Yosei ben Ḥalafta, but it is to Rabbi Yosei, son of Rabbi Yehuda, who said: Her latest immersion is sufficient. A woman who is uncertain with regard to the correct time for her immersion need not immerse multiple times. She may postpone her immersion until a time when she will be certain to fulfill her obligation, even though it might not be immersion at the designated time.

Rabbeinu Tam rules that immersion at the proper time is not a mitsvah...and Rabbeinu Chananel rules...that immersion at the proper time is a mitsvah.

If her husband is in the city: This is even according to the opinion that says that immersion at the proper time is not a mitsvah, because nonetheless procreation is a mitsvah.

And that which is written in the name of Rabbeinu Tam that immersion in the proper time is not a mitsvah....because anyway all of our women do not immerse after the seventh day...

And in any case, it seems that if her husband is in town, it is a mitsvah to immerse at the proper time because we saw that Joshua was punished for preventing the people from engaging in procreation for one night. If she delays immersion to upset her husband, that is a sin...